1. Field of the Invention
The present invention relates to integrated solar cells, particularly amorphous silicon semiconductor solar cells formed on a flexible ribbon-like substrate.
2. Description of the Related Art
Amorphous silicon semiconductor layers have been widely studied for use as a semiconductor layer for a solar cell, since they can be deposited uniformly in a large area onto a substrate at a low temperature by glow discharge decomposition of silane gas or the like and since various substrates such as glass, polymer films ceramic plates, and metal foils may be used. As a basic structure of an amorphous silicon solar cell, a laminate of a metal electrode layer/an amorphous silicon semiconductor layer/a transparent electrode layer formed on the above-mentioned various substrates is known.
It is easy to deposit an amorphous silicon layer onto a ribbon-like substrate having a metal electrode layer by utilizing the above-mentioned features of amorphous silicon film deposition and by using a roll-to-roll process disclosed in Japanese Unexamined Patent Publication (Kokai) No. 59-34668 and U.S. Pat. No. 4,438,723 or by a three-chamber separated formation process described in the Japanese Journal of Applied Physics, Vol. 21, No. 3, p. 413 (1982). It is also easy to form a transparent electrode layer onto the amorphous silicon semiconductor layer by depositing a transparent conductive oxide.
To use the resultant laminate as an electric power supply, provision of lead terminals to the metal electrode layer and the transparent electrode layer is necessary. Further, to obtain a higher output voltage necessary for practical use, division of the laminate or a solar cell into unit cells and series connection of the unit cells by electrically connecting a metal electrode layer of a unit cell to a transparent electrode layer of the neighboring unit cell are necessary, because the output voltage of such a solar cell is in a range of about 0.6 to 5 V irrespective of its area. In these cases, ordinarily, the metal electrode layer or the bottom electrode layer of the laminate is first exposed and then connected to the transparent electrode layer or the top layer of the laminate. To expose the metal electrode layer, the following processes are used or proposed.
(a) Use of a metal mask during deposition of the amorphous silicon layer (Kausche et al, U.S. Pat. No. 4,245,386).
(b) Removal of a part of the amorphous silicon layer by a wet or dry etching process after deposition of the amorphous silicon layer.
(c) Removal of a part of only the amorphous silicon layer by irradiation of a laser beam to melt and evaporate it after deposition of the amorphous silicon layer (S. Yamazaki et al, "Mask-Less Fabrication of a-Si Solar Cell Using Laser Scribe Process", IEEE Photovoltaic Specialist Conference, May 1984, pp 206-211).
Among the above processes, process (a) is not suitable for a roll-to-roll processing or a large area processing. Even in an amorphous silicon deposition process, process (a) does not give a good pattern and cannot easily expose partially the surface of the metal electrode layer in an electrically satisfactory state, because heating during deposition of the amorphous silicon prevents good contact between the substrate and the mask due to the difference of thermal expansion coefficients and as a result amorphous silicon is deposited in the space between the substrate and the mask. Further, it is difficult to control the deviation of alignment to within about 0.5 mm in mask-alignment.
Process (b) can be used by a combination of resist coating and etching, but is not suitable for manufacturing solar cells at a low cost in mass production because it needs many steps such as coating the resist, exposure, washing, and etching.
Moreover, integrated solar cells produced by the above-mentioned process (a) or (b) need a large area for a connection portion for connecting unit cells. This reduces the active area and thus area efficiency of the integrated solar cells.
Use of a laser beam to divide a laminate or a large solar cell into unit cells allows selection of the width of the groove dividing the cells in a range between several tens to several hundreds of micrometers by control of the optical system and allows accurate division into the unit cells in desired patterns by a computer control system having a program for the desired patterns. Further, by moving mirrors or optical glass fibers in an optical system, division of a solar cell on a wide, continuously running ribbon-like substrate is possible. Thus, process (c) allows good productivity in division of a solar cell.
We tried to use a laser beam to divide a solar cell. As a result, we found that the metal electrode layer was damaged by irradiation of a laser beam necessary for melting and evaporating the silicon layer on the metal layer, even when the metal electrode layer was of a high melting point metal. This phenomenon disturbed the electrical surface state of the metal electrode layer.
If the metal electrode layer was of a low melting point metal, selective removal of the silicon layer was impossible. Moreover, heat damage to the amorphous silicon layer occurred around the portion where the laser beam was irradiated. Particularly, we found by Raman spectrometry that crystallization of the amorphous silicon layer occurred there. If crystallization occurred in the silicon layer, the dark conductivity was increased at the crystallized portion and pin-type junctions at that portion were destroyed, resulting in disappearance of the rectification effect. The electromotive force generated in the solar cell was therefore lost in the crystallized portion after the laser scribing process. Furthermore, observation of a section of the divided portion with a scanning electron microscope revealed that the bottom metal electrode layer and the top transparent electrode layer were electrically joined by fusion thereof. This was also a cause of deterioration of the characteristics of the solar cell.